A heater wire known in the art (see, for example, Patent Document 1) is prepared as follows. A first heater wire is prepared by spirally winding a rectangular wire around a core wire and forming a meltdown layer around these wires, a second heater wire is prepared in the same manner as the first heater wire, the first heater wire and the second heater wire are twisted together, a signal wire is spirally wound around these twisted wires, and an insulating sheath is formed on the peripheral surface of these wires.